There is increased interest in light emitting diode displays made from organic materials because of their relatively low cost, ability to emit light, low power consumption, low driving voltages, ability to view the image at 180 degree angles, and good color tenability that allows them to be used in full color displays. The organic materials are deposited by, for example, spin-coating or ink-jet printing (in the case of polymer materials), or by evaporation (in the case of small organic molecules). Ink-jet printing is becoming an increasingly attractive organic material dispensing technique as displays are moving toward color displays and display manufacturing technologies are being driven toward further automation, miniaturization, and reductions in costs, cycle times, and environmental impact. Advantages offered by ink-jet printing include low-cost, precise control of dispensed volumes, data-driven deposition, and environmental friendliness.
Ink-jet printing systems are used to manufacture organic light emitting diode displays. With these ink-jet printing systems, the positions within a pocket at which a droplet of an organic material is to be released from one or more nozzles of a print head is specified by writing in a file the distances (e.g., in millimeters) from the center of the pocket at which the droplet of the organic material is to be released. Specifying the distance from the center can be time consuming, can be cumbersome, and can be inaccurate due to human error. Also, by merely specifying distances from the center at which droplets of organic material are to be released, a user is not given a preview of an approximation as to how the specified droplets will fill the pocket and whether, for example, the droplets will overflow the pocket or will be evenly distributed within the pocket. By not providing this preview to the user, the user will have more difficulties in choosing the best printing pattern and the locations at which the droplets are released in order to improve efficiency, lifetime, and printability. By having to perform the time consuming task of measuring and specifying the distances from center, more time is needed to change and create a new printing pattern for testing.
With typical ink-jet printing systems, adjacent nozzles do not deposit organic material in pockets of corresponding adjacent lines of the display (e.g., nozzle 1 does not deposit organic material into a pocket in line 1 and nozzle 2 does not deposit organic material into a pocket in line 2) because the strong rotation of the print head needed to fill these pockets can lead to errors such as droplets being deposited outside the pocket boundary. After the first print cycle (e.g., after the first print phase), these ink-jet printing systems come to a halt. The operator typically has to manually instruct these ink-jet printing systems to perform one or more additional cycles (i.e., perform additional print phases) so that all of the pockets of the display are printed. Manually performing this printing process makes this a time consuming and operator-intensive task. In addition, with these ink-jet printing systems, the adjacent lines are typically printed using the same nozzle. For example, to print a polyethylenedioxythiophene (“PEDOT”) layer, three print cycles (i.e., phases) are needed to print groups of three adjacent lines because of the angle error resulting from the rotation of the print head as described earlier. When employing this process of using three print cycles to print the display, each group of three adjacent lines are printed using the same nozzle. Since each of the lines in the group are printed using the same nozzle, the lines having the same concentrations of organic material are grouped together. This grouping results in more noticeable differences in the brightness of the display at, for example, the boundaries of the groups. In this case, the differences (i.e., errors) in the concentrations of the lines are not randomly dispersed.
With the typical ink-jet printing systems, during a single print cycle, two or more different nozzles cannot be used to release droplets of organic material into the same pocket of a particular line. This can result in large differences between lines as to the amount of organic material contained in the pockets of the lines due to differences in the amount of organic material released by different nozzles. For example, if the first nozzle of the print head releases 1.2 picoliters and the fifth nozzle of the print head releases 0.8 picoliters, then the pockets filled with either of these nozzles contain 1.2 picoliters or 0.8 picoliters of organic material. This difference in the amounts of organic material deposited results in, for example, decreased display quality because the pixels (i.e., pockets) are not emitting at uniform brightness. Also, the pixels containing lesser amounts of organic material generally have a shorter lifetime. With these ink-jet printing systems, the pockets of a particular line can be printed by two or more different nozzles only by reprinting on the same display after the first print iteration completed. But by the time the display is ready for reprinting, the organic materials deposited by the previous print iteration have already dried, resulting in the droplets deposited by the current iteration forming a new layer rather than combining with the droplets deposited from the previous print iterations to form homogeneous films at the pockets.
For the foregoing reasons, there exists a need to accurately, quickly, and conveniently specify the encoder steps at which droplets of an organic material are to be deposited from the nozzles of the print head. Also, there exits a need to quickly and efficiently print all the pockets of the display and print the pockets such that adjacent lines within the set are printed using different nozzles. There also exists a need to deposit droplets of an organic material from two or more different nozzles into a single pocket during one print iteration.